1. Field of the Invention
The present invention relates to an electron beam drawing system, and more specifically to an electron beam drawing control system of the raster scanning type which enables high speed drawing.
2. Description of the Prior Art
FIG. 1 is a block diagram illustrating a general arrangement of a known raster scanning electron beam drawing control system.
In FIG. 1, a disc memory 100 stores condensed, parametric, data for a number of basic figures which may be drawn in basic drawing areas called cells. The condensed data is produced in advance from a whole pattern to be drawn.
A controller 102 controls the delivery of condensed data stored in disc memory 100 to a high speed data transfer unit 104.
A pattern generating unit 106 produces dot pattern data from the condensed data. Number 108 denotes a memory having memory units the size of cells for storing the dot pattern data delivered from pattern generating unit 106. Numeral 110 is an electro-optical device for directing electron beam 11 onto a sample 112 mounted on a table 114 which is moved by a table drive circuit 116. Numeral 118 is a drive circuit for operating electro-optical device 110.
FIG. 2 illustrates the relationship between electron beam 111 emitted from raster scanning electro-optical system 110 and a sample 112 on Table 114. FIG. 3 is an enlarged and detailed perspective view of FIG. 2. As can be seen from FIGS. 2 and 3, electron beam 111 is deflected across the cell area which has a width "d" corresponding to 512 bits in the X direction of each cell. The electron beam is switched ON and OFF corresponding to a dot pattern during a single scan across the cell. Table 114 is adapted to move at a constant speed in the direction of Y such that after scanning in the Y direction, a subsequent scanning for the adjacent column proceeds.
FIG. 4 is a more detailed block diagram of the conventional control system in FIG. 1. An interface 120 controls a pre-processing unit (PPU) 124, a function generator (FG) 126, a write control unit (WCU) 128, and a read control unit (RCU) 130 through control bus 122.
A high speed data transfer unit (DMA) 132 transfers a few units of condensed data to a data memory 134 through data bus 136. Each unit of condensed data for each cell is supplied through controller 102.
Pre-processing unit 124 receives condensed, parametric data DA identifying the shape and location of figures to be drawn (height, width, starting location, etc.) in a shortened notation from data memory 134 through data bus 136, and transforms the data so that it is suitable for function generator 126.
Function generator (FG) 126 generates raster blanking data for the electron beam from the above-mentioned compressed data DA. The blanking data includes the commands for activating or deactivating the electron beam, in addition to data related to the address at which the command is to be effected. Write control unit (WCU) 128 generates both addresses within cell memories 138, 140 and 142 and 16-bit blanking data corresponding to those addresses based on blanking data and address data given by function generator (FG) 126. The blanking data is written in the appropriate cell memory 138, 140 or 142 at the selected address. Each cell memory 138, 140 or 142 is organized to have 512 rows each with 32 bytes of 16 bits each (512 bits.times.512 bits). This corresponds exactly to the size of one cell.
PPU 124, FG 126 and WCU 128 will be referred to below as dot pattern converting unit (DPCU) 144 which transforms compressed data DA corresponding to figures to be drawn into blanking data. Read control unit 130 transforms blanking data from cell memories 138, 140 and 142 into serial data, and supplies the serial dot data to electro-optical system 110 in FIG. 1.
Data buses 146 and 148 connect cell memories 138, 140 and 142 to WCU 128 and RCU 130. RCU 130 reads out cell memories 138, 140 and 142 serially and cyclically. As data is being read from one cell memory, data is also being written into a different cell memory. For example, the cell memories are controlled such that writing in cell memory 140 occurs simultaneously with reading from cell 142, and writing in cell memory 140 by WCU 128 is completed at least before the contents of cell memory 142 are read by RCU 130. RCU 130 can thus transfer dot data smoothly with the three cell memories 138, 140 and 142.
Typically, data is transferred from RCU 130 to electro-optical system 110 at a speed of around 20 MHz at an electron beam spot diameter of 1.mu.. Recently, it has been desired to improve the resolution of display patterns by reducing the diameter of the electron beam spot to 0.5.mu..
However, in order to display a line having the same width as the conventional one at the higher resolution, twice as many bits must be scanned as compared to the conventional case, which would require twice the time unless the scanning speed is doubled. Unless the speed is doubled, the drawing system would need to operate for 20 to 30 hours for a complicated drawing pattern.
In the control system shown in FIG. 4, it takes from 2 to 13 milliseconds for the dot pattern converting unit DPCU 144 to process data for one cell. Transferring condensed data from data memory 134 to DPCU 144 takes on the order of 2 to 3 microseconds, resulting eventually in a serial data transfer rate from RCU 130 limited to 20 MHz due to the relatively slow processing time by FG 126 and WCU 128 in DPCU 144.